This disclosure relates generally to the field of semiconductor diagnostics. More particularly, but not by way of limitation, this disclosure relates to devices, systems and methods for probing integrated circuits using laser illumination.
When an incident laser is focused onto an integrated circuit (IC), the device under test (DUT), free-carriers within the device absorb and refract photons supplied by the laser. As a result, the amplitude modulation of the reflected laser light corresponds to the DUT's response to an applied electrical test pattern. Analysis of the reflected laser light reveals direct information about the active DUT. Two traditional techniques, laser voltage imaging (LVI) and laser voltage probing (LVP), are used during IC debugging operations.
LVI relies on a spectrum analyzer, a lock-in amplifier or a similar device to map a specified frequency onto a laser scanning microscope (LSM) image of the DUT. As the laser is raster-scanned over the active DUT, the amplitude of the reflected light is modulated and corresponds to the DUT signal (captured by, for example, a photodetector). The DUT signal is supplied to a spectrum analyzer or similar device which is set to the exact frequency of interest (zero-span mode). The output voltage of the spectrum analyzer is directly proportional to the strength of the signal at the specified frequency. In other words, when the specific frequency component of the signal is small or nonexistent, the spectrum analyzer's output voltage level falls into the noise floor of the DUT signal. When the frequency component is large, the spectrum analyzer's voltage response increases. A frame-grabber receives the spectrum analyzer output signal. On a separate channel, the frame-grabber simultaneously creates the LSM image. Each time the LSM steps, a spectrum analyzer measurement is made. The resulting LVI map perfectly overlays the LSM image. In other words, the spectrum analyzer output generates a map of the scanned area that displays gray-scale levels corresponding to the device activity—at a given frequency—at each point of the LSM field of view. Several spectrum analyzers placed in parallel allow multiple frequencies (one frequency per spectrum analyzer) to be monitored simultaneously. Further alterations to the system—for example, replacing the spectrum analyzer with a lock-in amplifier—yield logic-state maps (phase maps) in addition to the frequency maps discussed above.
LVP, in contrast, yields waveforms from a specific location within the LSM field of view. That is, waveform collection takes place after the LSM image is acquired and the scanning has stopped. Specifically, individual DUT sites within the LSM field of view are manually probed by parking the laser on the area of interest. In operation, the reflected laser light is converted into an AC signal (e.g., by a photodetector), amplified and sent to an oscilloscope (the oscilloscope's trigger or sync signal must be synchronous with the test pattern applied to the DUT). The resulting waveform contains both timing and frequency information from the DUT at the probe location. During a typical debug procedure, large numbers of waveforms are recorded. Such probing is a deliberate and time-consuming operation. Because of this, only suspect locations on the DUT are typically examined.
Both LVI and LVP techniques suffer from limitations. Sufficient detection of the DUT's output signal (i.e., the reflected laser light) for LVI depends on the nature of the signal itself. In general, the duty-cycle and periodicity of a signal determines the spectral response of that signal. As the signal becomes less ideal—i.e., deviates from 50% duty-cycle—the number of spectral components increase which makes the response at the fundamental frequency (or any component of interest) less prominent. Consequently, the LVI signal decreases. Because LVI necessitates a robust signal, only periodic signals with sufficient duty-cycle are detectable. Imaging pulsed signals, or pulse-trains, can be exceedingly difficult or impossible. In the case of LVP, because waveforms are recorded with an oscilloscope, any repetitive signal with minimal jitter suffices. As a point measurement technique however, LVP requires deliberate waveform collection at individual probe sites (waveform collection requires that the applied laser is stationary at the measurement site; not scanning). While powerful, at the cost of minutes per waveform—including optimizing the signal and probe location—LVP is inefficient.